The invention relates to a special device for the purification of diesel exhaust gases, which device comprises, in the flow direction of the exhaust gas, an oxidation catalyst, a diesel particle filter with catalytically active coating, and, downstream of a device for introducing a reducing agent from an external reducing agent source, an SCR catalyst.
The untreated exhaust gas of diesel engines contains, in addition to carbon monoxide CO, hydrocarbons HC and nitrogen oxides NOx, a relatively high oxygen content of up to 15% by volume. Said untreated exhaust gas also contains particle emissions which are composed predominantly of soot residues and possible organic agglomerates and which arise from partially incomplete fuel combustion in the cylinder.
Adhering to future legal exhaust-gas limits for diesel vehicles in Europe, North America and Japan, necessitates the simultaneous removal of particles and nitrogen oxides from the exhaust gas. The harmful gases carbon monoxide and hydrocarbons from the relatively lean exhaust gas can easily be made harmless by oxidation at a suitable oxidation catalyst. Diesel particle filters with and without an additional catalytically active coating are suitable units for the removal of the particle emissions. On account of the high oxygen content, the reduction of the nitrogen oxides to form nitrogen (“denitrogenization” of the exhaust gas) is more difficult. A known method is selective catalytic reduction (SCR) of the nitrogen oxides at a suitable catalyst, SCR catalyst for short.
Said method is presently the preferred option for the denitrogenization of diesel engine exhaust gases. The reduction of the nitrogen oxides contained in the exhaust gas takes place in the SCR method with the aid of a reducing agent which is introduced into the exhaust strand in a dosed fashion from an external source. As reducing agent, use is preferably made of ammonia or of a compound which releases ammonia, such as for example urea or ammonium carbamate. The ammonia, which is possibly generated in situ from the precursor compound, reacts at the SCR catalyst with the nitrogen oxides from the exhaust gas in a comproportionation reaction to form nitrogen and water.
At present, in order to satisfy the upcoming legal standards, a combination of the different exhaust-gas purification units is inevitable. A device for the purification of diesel engine exhaust gases must comprise at least one oxidationally active catalytic converter and, for denitrogenization, an SCR catalyst with an upstream device for introducing reducing agent (preferably ammonia or urea solution) and an external reducing agent source (for example an auxiliary tank with urea solution or an ammonia store). If it is not possible by optimizing the combustion within the engine to keep the particle emissions sufficiently low that they can be removed by means of the oxidation catalyst by direct oxidation with oxygen, the use of a particle filter is additionally necessary.
Corresponding exhaust-gas purification systems have already been described; some are presently at the practical testing stage.
For example, EP-B-1 054 722 describes a system for the treatment of NOx and particle-containing exhaust gases, in which system an oxidation catalyst is connected upstream of a particle filter. Arranged at the outflow side of the particle filter are a reducing agent source and a dosing device for the reducing agent, and an SCR catalyst. In the method described here, the NO2 proportion in the exhaust gas and therefore the NO2/NOx ratio is increased by means of the at least partial oxidation of NO at the oxidation catalyst, with the NO/NO2 ratio preferably “being set to a predetermined level which is an optimum for the SCR catalyst”.
Said NO/NO2 ratio which is an optimum for the SCR catalyst is 1 for all presently known SCR catalysts. If the NOx contained in the exhaust gas is composed only of NO and NO2, then the optimum NO2/NOx ratio is between 0.3 and 0.7, preferably between 0.4 and 0.6 and is particularly preferably 0.5. Whether said ratio is attained upstream of the SCR catalyst in a system according to EP-B-1 054 722 is dependent on the exhaust-gas temperature and therefore on the operating state of the engine, on the activity of the oxidation catalyst and on the design and soot loading of the diesel particle filter which is connected downstream of the oxidation catalyst.
The untreated exhaust gas of conventional diesel engines contains only a very low proportion of NO2 in the NOx. The main proportion of the nitrogen oxides is nitrogen monoxide NO. As said untreated gas passes over the oxidation catalyst, NO is at least partially oxidized to form NO2. The rate of NO2 formation is dependent on the activity of the oxidation catalyst and on the exhaust-gas temperature. If a significant quantity of soot is deposited on the diesel particle filter which is arranged at the outflow side, then the NO2 proportion present in the NO downstream of the oxidation catalyst is, with sufficient exhaust-gas temperature, further reduced. Since NO is predominantly formed from the NO2 during the oxidation of soot with NO2, however, no denitrogenization of the exhaust gas takes place as a result. Said denitrogenization must take place by means of the downstream SCR catalyst, for which purpose the NO2/NOx ratio must be set to an optimum value over the entirety of oxidation catalyst and diesel particle filter. EP-B-1 054 722 however does not provide any technical teaching as to how said setting of the NO2/NOx ratio in the exhaust gas upstream of the SCR catalyst can be realized over the entirety of the oxidation catalyst and filter.